Hematopoietic growth factors are the major regulatory molecules supporting constitutive and inducible hematopoiesis (Brach et al. Acta. Haematol. 86, 128 (1991)). The hematopoietic growth factors (colony stimulating factors and interleukins), growth-factor synergizing factors, and growth factor-releasing factors control the proliferation, differentiation, and functional activation of hematopoietic stem cells and lineage-committed progenitor cells. Each colony stimulating factor has distinct lineages of bone marrow cells upon which they act, although there is some overlap in lineage activity and synergy between colony stimulating factors. In several instances, the involvement of growth factors in the maturation of specific hematopoietic cell types is well known, as in the action of erythropoietin to produce erythrocytes and granulocyte colony stimulating factor to produce neutrophils. However, there are a number of stages in hematopoietic cell development where the identification of stimulatory factors is incomplete or lacking altogether. This is particularly true for those events leading to the proliferation and development of early hematopoietic progenitor cells.
Hematopoietic progenitor cells develop gradually from pluripotent to unipotent, committed progenitor cells during which process they lose their self-renewal capacity (Olofsson Aca. Oncol. 30, 889 (1991)). This development is dependent on interactions of specific hematopoietic growth factors, which by binding to surface receptors on the stem cells stimulate them to proceed to the next step of differentiation. Interleukin-3 (IL-3) is primarily a proliferative stimulus for the undifferentiated progenitor cells (Ponting et al. Growth Factors 4, 165 (1991)). Granulocyte Macrophage-Colony Stimulating Factor (GM-CSF) also plays a major role in multipotent stem cell survival, proliferation and differentiation into stem cells with restricted maturation programs. The programmed unipotent stem cells need stimulation by erythropoietin, granulocyte-colony stimulating factor (G-CSF), macrophage colony stimulating factor (M-CSF) and IL-5 to proliferate and mature into their end stage products, erythrocytes, neutrophils, monocytes and eosinophils respectively. Other cytokines such as IL-1.beta., IL-4 and IL-6 fulfill important functions as cofactors in these processes (Arai et al. Ann. Rev. Biochem. 59, 783 (1990)).
Stem cell factor (SCF), also referred to as the ligand for c-kit, was recently identified as a cytokine which stimulates the proliferation of progenitor cells (PCT Application No. WO 91/05795). SCF has the capacity to synergize with a wide variety of other hematopoietic growth factors to cause the proliferation and differentiation of committed progenitor cells (Migliaccio et al. J. Cell Physiol. 148, 503 (1991)). In clonal cultures of normal mouse marrow cells, combination of G-CSF, GM-CSF or IL-3 with SCF induced up to 25 fold increase in the mean cell content and up to 6-fold increase in their mean progenitor cell content (Metcalf Proc. Natl. Acad. Sci. USA 88, 11310 (1991)).
Progenitor cells committed to the lymphoid lineage eventually mature to B or T lymphocytes. Mature B cells mediate humoral antibody responses by producing antibodies which circulate in the bloodstream and bind foreign antigens. The binding of antigen by antibody leads to antigen destruction by phagocytosis or by activation of complement. Antibody-producing B cells comprise a major part of the human immune response.
The involvement of growth factors in the proliferation and differentiation of hematopoietic progenitor cells to mature B cells is essential for maintaining B cell levels. The identification of such factors will be important in developing therapeutic strategies for modulating B cell levels, particularly in immunodeficient patients. One area of research is the identification of factors acting early in B cell development to stimulate the production of B cell progenitors such as pre-B cells. Pre-B cells are characterized as the early progenitor cells which express the .mu. heavy chain of immunoglobulin in their cytoplasm but do not express cytoplasmic light chain or surface immunoglobulin.
U.S. Pat. No. 4,965,195 disclosed that interleukin-7 (IL-7) stimulates the proliferation of pre-B cells derived from mouse bone marrow. McNiece et al. (J. Immunol. 146, 3785 (1991)) showed that SCF interacts synergistically with IL-7 to stimulate proliferation of B lineage cells. However, the requirement for additional factors in B cell formation has been suggested by the work of Billips et al. (Blood 79, 1185 (1992)). The Billips et al. reference demonstrates that pre-B cell formation from B220-, Ig-progenitor cells and expression of .mu. heavy chain of immunoglobulin is uniquely dependent on the presence of S17 stromal cells and can not be reproduced with IL-7, SCF, or costimulation with both IL-7 and SCF. In addition, stromal derived lymphopoietic factor-1 (SDLF-1) that alone stimulates the differentiation of B progenitor cells into pre-B cells has been described (PCT Application No. WO 89/06541).
It is therefore an object of the invention to identify factors that are involved in promoting the proliferation and differentiation of hematopoietic progenitor cells, particularly lymphoid progenitor cells, into B lineage committed cells such as pre-B cells. The factors of the invention are useful as modulators of the humoral antibody response. The therapeutic benefit of factors acting to stimulate B cell progenitors makes it desirable to identify and express the genes encoding said factors.